Gala has this https://github.com/donadigo/gala-pip-plugin plugin by default. How do I use/activate it?
I'm not actually sure if Gala is updated on elementary OS Loki, so if the following steps don't work you might need to build gala from source to get the latest version.
When you press the combination of Windows+D (windows key is also known as super) your cursor will turn into a
In that mode you can click and drag a blue box with the contents that you want to see in the popup window.
To resize the window, go to the bottom right of the popup window, press the icon that appears and drag the window as you please.
To close it, go to the top left corner of the popup and click on the X button that appears.